1. Field of the Invention
The present invention relates to a measurement apparatus for measuring the shape of a test object and a measurement method.
2. Description of the Related Art
A light wave interferometer using the interference of light generally has a high measurement accuracy, so even a weak vibration occurring during measurement has large influence on the measurement result. Therefore, demands have arisen for reducing measurement errors caused in the light wave interferometer by vibrations. Japanese Patent Laid-Open No. 8-219738 has disclosed a method of detecting a relative vibration between a reference surface and test surface by using an acceleration sensor, and moving at least one of the reference surface and test surface so as to cancel the fluctuation in the relative position, thereby reducing measurement errors caused during measurement by vibrations.
In Japanese Patent Laid-Open No. 2000-275021, a measurement error caused on the shape of a test surface by a disturbance vibration is corrected by using the property that when a phase error caused by a disturbance vibration is much smaller than a phase shift amount, a phase φ′ calculated by the phase shift method is approximately represented by:φ′=φ+a·cos(2φ)+b  (1)where φ is the true value of a phase. Also, each of an amplitude a and constant term b changes in accordance with the vibration state during measurement, and has a different value whenever measurement is performed.
In Japanese Patent Laid-Open No. 2000-275021, an interference signal generated by a reflected light beam from an error detection surface positioned around a test surface and a reflected light beam from a part of a reference surface is measured simultaneously with an interference signal generated by the test surface and reference surface, and the phase distribution on the error detection surface is measured by the phase shift method. Then, the coefficients a and b in equation (1) are calculated from the measured phase distribution on the error detection surface, and a measurement error generated on the shape of the test surface by a disturbance vibration is corrected by using the coefficients a and b.
In Japanese Patent Laid-Open No. 8-219738, however, a plurality of acceleration sensors are necessary to directly detect the relative vibration between the reference surface and test surface. This increases the apparatus cost compared to ordinary interferometers. In Japanese Patent Laid-Open No. 2000-275021, the apparatus cost does not largely increase because a measurement error caused by a vibration can be measured by adding only the error detection surface. In Japanese Patent Laid-Open No. 2000-275021, however, correction is performed using an approximate expression represented by equation (1), so it is impossible to accurately correct a measurement error caused by a disturbance vibration if the vibration amount is large.